Preheat the oven to 375°F. Bring 3 to 4 quarts of water to boil in a pot. Add 1 tablespoon salt and the pasta. Cook, stirring a few times, until just tender but still quite firm (very al dente). Drain and rinse under cool water.

Melt 2 T. of the butter in a Dutch oven over medium heat. Add the breadcrumbs and Parmigiano-Reggiano and stir well, about 1 minute. Spoon the mixture into a bowl and wipe out the pan. In the same Dutch oven, melt the remaining 2 T. of butter over medium heat. Add the pancetta and sauté over medium heat, until it’s almost crisp, then add the onion and cook, stirring occasionally, until soft and translucent, about 5 minutes.

Slowly whisk in the flour and stir until the mixture starts to colour slightly and smell a bit nutty, about 3 minutes more. Add the mustard powder, cayenne and nutmeg. Stir well to incorporate.

In a medium bowl, mix the milk, cream and buttermilk together and gradually add to the pancetta/onion mixture, whisking to incorporate. Stir until thick enough to see streaks on the bottom of the pan. Gradually add 1-1/2 c. of the cheddar and all of the gruyère. Stir until the mixture is thick and all the cheese has melted, about 2 minutes.

Add the reserved pasta to the Dutch oven and stir well. Smooth the top and sprinkle with the remaining cheddar. Spread the breadcrumb mixture evenly on top. Bake until bubbling and golden brown, about 30 minutes. Let cool about 10 minutes before serving. Serves 6.